Internal combustion engines include intake valve and exhaust valves. The opening and closing of these valves are synchronized with the rotation of a crankshaft of the internal combustion engine and performed at appropriate timings. Specifically, there are configurations in which when the crankshaft rotates, a camshaft coupled to the crankshaft rotates, and a plurality of cams provided on this camshaft open and close respective valves.
Even if the opening and closing timings of the valves, i.e., the crank angles at which the opening and closing of the valves is performed, are always constant, the internal combustion engine may be operated. However, the optimal opening and closing timings of the valves are not always the same, and may change according to operating conditions of the internal combustion engine (such as rotation speed or torque).
For this reason, there have been developments in systems which are able to change the opening and closing timings of valves according to operating conditions, i.e., variable valve systems, and such systems have already been implemented. A variable valve system may change a relationship between the rotation angle of the crankshaft (crank angle) and the rotation angle of the camshaft (cam angle), i.e., a camshaft phase representing a relative rotation angle between the crankshaft and the camshaft. As a result, the opening and closing timings of at least one of the intake valves or the exhaust valves may be changed (for example, see Patent Literature 1).
In a variable valve system, when controlling the opening and closing timings of the valves to be appropriate, it is necessary for the aforementioned camshaft phase to match a target value. For this reason, there is provided a sensor for measuring the crank angle and a sensor for measuring the cam angle. Then, based on the measurement values from these sensors, the camshaft phase at the present time is calculated.